The present invention relates to an electroconductive liquid crystal device, particularly a conductive liquid crystal device useful as an organic electroluminescence device (hereinafter, the term “electroluminescence” is sometimes abbreviated as “EL” according to common usage in the field).
As for the organic EL device, carrier injection-type EL devices utilizing organic solids, such as anthracene single crystal, were studied in detail in the 1960s. These devices were of a single layer-type, but thereafter Tang, et al. proposed a lamination-type organic EL device comprising a luminescence layer and a hole transporting layer between a hole injecting electrode and an electron injecting electrode. The luminescence mechanism in these injection-type EL devices commonly includes stages of (1) electron injection from a cathode and hole injection from an anode, (2) movement of electrons and holes within a solid, (3) recombination of electrons and holes, and (4) luminescence from the resultant single term excitons.
A representative example of the lamination-type EL device may have a structure including an ITO film as a cathode formed on a glass substrate, a ca. 50 nm-thick layer formed thereon of TPD (N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), a ca. 50 nm-thick layer thereon of Alq3(tris(8-quinolinolato)aluminum), and further a vacuum deposition layer of Al-Li alloy as a cathode. By setting the work function of the ITO used as the anode at 4.4-5.0 eV, the hole injection to TPD is made easier, and the cathode is composed of a metal which has as small a work function as possible and also is stable. Examples of the cathode metal may include Al-Li alloy as mentioned above and also Mg-Ag alloy. By the above organization, green luminescence may be obtained by applying a DC voltage of 5-10 volts.
An example using a conductive liquid crystal as a carrier transporting layer is also known. For example, D. Adam, et al. (Nature, vol. 371, p. 141-) have reported that a long chain triphenylene compound as a discotic liquid crystal material exhibited a mobility of 10−3-10−2 cm2/V.sec in its liquid crystal phase (Dh phase) and a mobility of 10−1 cm2/V.sec in its mesophase (an intermediate phase, not a liquid crystal phase). Also, as for a bar-shaped liquid crystal, Junichi Hanna (Ohyou Butsuri) (Appl. Phys. vol. 68, no. 1, p. 26-) has reported that a phenylnaphthalene compound exhibited a mobility of 10−3 cm2/V.sec or higher in its smectic B phase.
As a trial for using such a liquid crystal for electroluminescence, Ingah Stapff, et al. (Liquid Crystals, vol. 23, no. 4, pp. 613-617) have reported an organic EL device using a triphenylene-type discotic liquid crystal. Other reports are found in POLYMERS FOR ADVANCED TECHNOLOGIES, vol. 9, pp. 463-460 (1998), and ADVANCED MATERIALS, vol. 9, no. 1, p. 48-(1997).
Conventional organic EL devices have involved several problems attributable to the use of a low molecular weight compound in a crystal state. A first problem is that the efficiency of injection of electrons or holes from electrodes of ITO, etc., to the organic layers is low. This is due to a minute grain boundary in a crystal state of organic molecules which functions as a carrier conduction barrier. Accordingly, organic molecules in an amorphous state are generally used, though lower carrier injection efficiency results. This, however, is a major reason why an organic EL device cannot ensure a large current.
An organic material used in an organic EL device has an electronic structure providing a large energy gap of ca. 3 eV or larger, thermal excitation-type free electrons are not present in a conduction band, and a drive current (spatial charge restriction current) is principally supplied by injected carriers from the electrodes, so that a low carrier injection efficiency from the electrodes has been a serious problem. As the injection efficiency is low, a large voltage has to be applied in order to ensure a drive current, and the device layer thickness has to be lowered. These factors have caused difficulties, such as a short circuit between the electrodes and an increase in capacitive load.
As a second problem, an organic EL device is liable to be affected by invasive moisture which causes the deterioration of luminescence performance and drive performance, and thus poor durability. In an ordinary organic EL device, the organic layers are disposed in lamination, and then the cathode is formed thereon by vapor deposition of a metal film. In this instance, a metal species having a small work function suitable for the cathode is susceptible to oxidation and has a low durability. Even when forming a protective film thereon by sputtering, the organic layers are liable to be degraded if the forming temperature is high (with an ordinary limit of 100° C.), and the destruction of the device structure due to the film stress is also problematic.
On the other hand, as a problem accompanying the use of a conducive liquid crystal to constitute a carrier transporting layer, it is difficult to align a high order liquid crystal layer. As a high order conductivity can be attained by a regular stacking of π-electron conjugated planes of liquid crystal molecules, the degree of alignment thereof directly affects the conductivity of the resultant conductive liquid crystal layer. In the case of a poor alignment characteristic, trap sites for conduction of electrons and holes can be formed in the liquid crystal layer, so that electroconductivity can be lost completely in some cases.